Metal Supported Solid Oxide Fuel Cell

ABSTRACT

A process for forming a metal supported solid oxide fuel cell, the process comprising the steps of: a) applying a green anode layer including nickel oxide, copper oxide and a rare earth-doped ceria to a metal substrate; b) firing the green anode layer to form a composite including oxides of nickel, copper, and a rare earth-doped ceria; c) providing an electrolyte; and d) providing a cathode. Metal supported solid oxide fuel cells comprising an anode a cathode and an electrolyte, wherein the anode includes nickel, copper and a rare earth-doped ceria, fuel cell stacks and uses of these fuel cells.

PRIORITY

The present application claims priority to, and is a divisionalapplication of, U.S. patent application Ser. No. 14/053,301 titled“Metal Supported Solid Oxide Fuel Cell” of Leah, et al. filed on Oct.14, 2013, the disclosure of which is incorporated by reference herein,and claims priority under 35 U.S.C. §119 to GB Application No.1315746.6, filed Sep. 4, 2013.

FIELD

The invention relates to a metal supported solid oxide fuel cell (SOFC),to processes for forming the fuel cell and to uses thereof. Theinvention relates, in particular, to metal supported solid oxide fuelcells wherein the anode comprises nickel and copper.

BACKGROUND

A SOFC is an electrochemical device for the generation of electricalenergy through the electrochemical oxidation of a fuel gas (usuallyhydrogen-based). The device is generally ceramic-based, using anoxygen-ion conducting metal-oxide derived ceramic as its electrolyte. Asmost ceramic oxygen ion conductors (for instance, doped zirconium oxideor doped cerium oxide) only demonstrate technologically relevant ionconductivities at temperatures in excess of 500° C. (for cerium-oxidebased electrolytes) or 600° C. (for zirconium oxide based ceramics),SOFCs operate at elevated temperatures.

In common with other fuel cells, SOFCs include an anode where fuel isoxidised, and a cathode where oxygen is reduced. These electrodes mustbe capable of catalysing the electrochemical reactions, be stable intheir respective atmospheres at the temperature of operation (reducingon the anode side, oxidising on the cathode side), and be able toconduct electrons so the electric current generated by theelectrochemical reactions can be drawn away from theelectrode-electrolyte interface.

Finding materials with the relevant combination of properties for theanode has, in spite of extensive research, proved difficult. For manyyears, the state-of-the-art SOFC anode has consisted of a porousceramic-metal (cermet) composite structure, with nickel as the metallicphase and an electrolyte material (usually yttria or Scandia-stabilisedzirconia) as the ceramic phase, although less commonly doped ceria-basedelectrolyte materials such as gadolinia or samaria-doped ceria have alsobeen used. In this structure, the nickel performs the role of catalyst,and the volume fraction of nickel is high enough that a contiguous metalnetwork is formed, thus providing the required electronic conductivity.The electrolyte material forms a contiguous ceramic backbone to theanode, providing mechanical structure, enhancing the bond between theanode and the electrolyte and also extending the anode-electrolyteinterfacial region some distance into the anode.

A well-known limitation of these cermet anodes is that at cell operatingtemperature the metallic nickel in the anode is only stable in areducing atmosphere. This is normally provided by the fuel gases, sounder normal operation the anode is stable. However, should the supplyof fuel gas be interrupted with the SOFC at operating temperature, theatmosphere within the anode will become oxidising. Under theseconditions the metallic nickel will oxidise back to nickel oxide. Thisoxidation is associated with a volume increase of greater thanapproximately 40%, because the metallic nickel which has been formed bythe reduction of sintered nickel oxide does not oxidise back to the samemorphology as the original nickel oxide from which it was formed.Instead it generates mesoporosity, occupying a larger volume than theoriginal nickel oxide. This volume change on reoxidation can generatelarge stresses in the anode structure, which in turn can result incracking of the anode and potential destruction of the SOFC cell.

The inability of many SOFC cells to undergo multiple reduction-oxidation(REDOX) cycles without suffering damage of this type has been a majorfactor inhibiting the widespread commercial adoption of SOFC technologyfor power generation, as SOFC systems generally require the presence ofcomplex and expensive purge gas systems to maintain a reducingatmosphere over the anodes in the event of an unexpected fuelinterruption, for example due to a failure elsewhere in the system whichrequires an emergency shutdown of the system for safety reasons.

The problem of inadequate REDOX stability is particularly acute in anodesupported fuel cells, currently the most common form of SOFC cell. Anodesupport is beneficial as it allows a very thin (<20 μm) layer ofelectrolyte (such as stabilised zirconia) to be used, as the electrolyteis non-structural. This in turn allows operation at a lower temperaturerange than is the case for electrolyte supported cells (650 to 800° C.rather than 850 to 1000° C.). Because the resistance of the electrolyteto oxygen ion transport is inversely proportional to the electrolytethickness, in electrolyte supported fuel cells, the resistance caused bythe thickness of the electrolyte layer is overcome by increasingoperation temperatures, exploiting the exponential drop off inresistance with temperature. As thinner layers can be used in anodesupported cells, operation temperatures can be reduced, which isgenerally desirable as it facilitates the use of lower-cost materials inthe SOFC system, and reduces the rate of various material degradationmechanisms such as the oxidation of metallic components.

In spite of these advantages, as the anode is the structural support ofthe SOFC cell in an anode-supported cell, the cells are very prone tocatastrophic failure on repeated REDOX cycling, as stress-inducedcracking can result in the cell completely breaking up.

In spite of considerable efforts by developers, no alternative to nickelhas achieved widespread adoption, as no suitable material has yet beendeveloped which combines nickel's relatively low cost, high catalyticactivity for both electrochemical oxidation of hydrogen and steamreforming of hydrocarbon fuel feeds, and high electronic conductivity.

Gorte et. al. (US 2005/227133 A1, U.S. Pat. No. 7,014,942 B2) havereported the use of copper in a SOFC anode partially or completelysubstituted for nickel. Copper has advantages as an electronicallyconductive phase in the anode, notably that it does not catalyse theformation of carbon from hydrocarbon fuels. However it is a poorcatalyst for the electrochemical oxidation of hydrogen and steamreforming of hydrocarbon fuels, so in the copper anodes tested by Gorteet al., an additional catalyst such as ceria was required to achieveadequate electrode performance. The other issue with the use of copperin conventional SOFC applications is that both copper metal and copperoxide have low melting points (1084° C. and 1326° C. respectively).Cermet anodes are typically formed by sintering a mixture of the metaloxide powder and the electrolyte powder at 1200-1500° C. in air,followed by reduction of the metal oxide to the metal using hydrogen onfirst operation of the SOFC. This range of sintering temperatures iseither close to or above the melting point of copper oxide (nickel oxideby contrast melts at 1955° C.), leading to excessive sintering of thecopper oxide phase. Also conventional SOFC operating temperatures are inthe range 700-900° C., close to the melting point of metallic copper,which tends to result in sintering of the copper phase during SOFCoperation, potentially causing performance degradation. To address thisissue, Gorte et al. developed a method of adding the copper to the anodein a post-sintering infiltration step using solutions of copper saltswhich were dried and then calcined to decompose the salt to copperoxide, thereby avoiding the need to sinter copper oxide at hightemperatures. However, the infiltration step, whilst allowing the use ofcopper cermets, may be difficult to scale up to industrial production.Another issue with copper is that although less reactive than nickel, itwill still oxidise if exposed to an oxidising atmosphere at temperature,and thus a copper-based anode also lacks REDOX stability.

There are factors relating to the design of the SOFC which can helpmitigate the damaging effects of REDOX cycling, these include:

-   -   Not using an anode supported cell—the anode can therefore be        thinner; reducing the overall volume change through REDOX        cycling and the danger of catastrophic cracking.    -   Operating at a lower temperature—the rate of nickel oxidation        increases exponentially with increasing temperature, starting        at >300° C. The lower the temperature of operation, the less        risk of nickel oxidation and volume expansion. Further, nickel        particles tend to oxidise though a core-and-shell mechanism,        where the outer surface oxidises rapidly, but then the core of        the particle oxidises more slowly as this is diffusion limited.        Thus at lower temperatures, it is likely that only the outer        surface of the nickel particles in the anode will reoxidise, not        the entire particle and any volume change will be reduced.    -   Provide the anode with a contiguous ceramic ‘backbone’—As the        electrolyte-based ceramic phase used in SOFC anodes is largely        unaffected by changes in oxygen partial pressure, this part of        the anode will not change volume during REDOX cycles affecting        the nickel phase. Thus the structural integrity of the anode and        its bond to the electrolyte will be enhanced if there is a        sintered porous ceramic network within the anode.

A design of SOFC cell which has the potential to meet these criteria isthe metal-supported SOFC design disclosed by the applicant in GB 2 368450. This SOFC cell uses a ferritic stainless steel foil as a structuralsupport. The foil being made porous in its central region to allow fuelaccess to the anode. The active cell layers (anode, electrolyte andcathode) are all deposited on top of the substrate foil as films. Thismeans the anode only needs to be around 15 μm thick as it is not thestructural support for the cell. This cell also allows operation attemperatures in the range 450-650° C., much lower than standardoperating temperatures. This is achieved through the use ofpredominantly cerium oxide (ceria)-based ceramic materials such as CGO10(gadolinium doped-cerium oxide, for CGO 10—Ce_(0.9)Gd_(0.1)O_(1.95)) asthe oxygen ion conducting electrolyte, which have an intrinsicallyhigher oxygen ion conductivity than zirconia-based materials. A thinfilm of stabilised zirconia is deposited in the electrolyte to preventinternal short-circuiting of the cell due to the mixed ionic-electronicconductivity of ceria-based electrolytes, as disclosed in GB 2 456 445,but as the zirconia layer is so thin, its resistance to oxygen iontransport is sufficiently low that low-temperature operation is notprevented. The SOFC cell of GB 2 368 450 uses a porous metal-CGO10composite cermet anode fabricated as a thick film with a thicknessbetween 5 and 30 μm. The anode is generally deposited by screen-printingan ink containing metal oxide and CGO10 powders and formed into a porousceramic layer by thermal processing to sinter the deposited powderstogether to form a contiguous structure bonded to the steel substrate.

A limitation imposed by the deposition of the ceramic layers onto aferritic stainless steel support by conventional ceramic processingmethods is the maximum temperature to which the steel may be exposed inan oxidising atmosphere due to the formation of a chromium oxide scaleat high temperatures in an oxidising atmosphere. This upper limit issubstantially below the 1200-1500° C. typically used when sinteringceramics and so methods have been developed for sintering rare earthdoped ceria electrolytes to >96% of theoretical density at <1100° C.,facilitating the formation of the gas-tight layer desired (GB 2 368 450,GB 2 386 126 and GB 2 400 486).

Surprisingly, sintering a nickel oxide-rare earth doped ceria compositeanode at these temperatures has proved more difficult than sintering theelectrolyte. This is because composites of two different oxide materialshave been found to sinter more poorly than a single phase material. Thusnickel oxide or the ceramic alone will sinter adequately at thesetemperatures, but as a composite sintering in air can be poor, leadingto weak necks between particles and a weak ceramic structure. This canresult in cell failure as a result of REDOX cycling, as the weak bondsbetween nickel particles break as a result of the volume changes duringthe REDOX cycle. This can ultimately result in the catastrophic failureof the cell through delamination of the electrolyte from the anode.

In order to improve the REDOX stability of the cell, it is desirable tofind a means of enabling sufficient sintering of the cermet structure atthe temperature range at which it is possible to fire the ceramic layerson a steel substrate. It would therefore be advantageous to provide fora method of preparing a metal-supported SOFC in which the anode isstable to redox cycling, robust to a loss of reducing atmosphere atoperating temperature, and yet can be made using commercially viableproduction methods. The invention is intended to overcome or ameliorateat least some aspects of this problem and those described above.

SUMMARY

Accordingly, in a first aspect of the invention there is provided aprocess for forming a metal supported SOFC, the process comprising thesteps of:

a) applying a green anode layer including nickel oxide, copper oxide anda rare earth-doped ceria to a metal substrate;

b) firing the green anode layer to form a composite including oxides ofnickel, copper, and a rare earth-doped ceria;

c) providing an electrolyte; and

d) providing a cathode.

The presence of the copper in the anode layer, generally as copperoxide, provides an anode with improved sintering between the nickeloxide and the rare earth-doped ceria. This in turn enhances theformation of the ceramic backbone in the anode and increases thestability of the anode (and fuel cell as a whole) to REDOX cycling asthe anode microstructure is more robust than if copper were absent andless prone to volume change during the reduction of nickel and copperoxide to nickel and copper on first use of the fuel cell, or during anychange on reoxidation if the reducing atmosphere is lost at operatingtemperatures, for instance in the event of unplanned system failure andloss of fuel supply.

In many cases, the process of the invention will further comprise thestep of compressing the green anode layer at pressures in the range 100to 300 MPa. This compression step increases the density of the of theunsintered green anode layer, ensuring that the particles of nickeloxide, copper oxide and rare earth-doped ceria are in sufficiently closecontact to sinter effectively at the temperatures employed in theprocess of the invention. It will often be the case that the compressionstep is used in combination with a step of heating the printed layer toremove residual organic materials from the ink base prior tocompression, to leave a green anode layer comprising nickel oxide,copper oxide and a rare earth-doped ceria that may be compressed.

The first step of the process as described is the application of a greenanode layer to the metal substrate, typically the metal substrate willbe a stainless steel substrate, in particular a ferritic stainless steelsubstrate, as ferritic stainless steel forms a chromium oxide surfacepassivation layer when heated. This passivation layer protects the bulkmetal of the support and provides a diffusion barrier between the anodeand the bulk metal of the support. As used herein, the terms “support”and “substrate” as referring to the metal support/substrate are intendedto be used interchangeably. The formation of a chromium oxidepassivation layer, as opposed to aluminium oxide or silicon oxidescommonly formed with other heat resistant steels, has the benefit thatchromium oxide is an electronic semi-conductor at high temperatures,rather than being insulating, making the ferritic stainless steelsuitable for use in fuel cell applications. The ferritic stainless steelmay be an aluminium free ferritic stainless steel, such as a ferriticstainless steel containing titanium and/or niobium as stabilisers. Oftenthe ferritic stainless steel will comprise from about 17.5 to 23 wt %Cr. In particular, the ferritic stainless steel may be selected fromEuropean designation 1.4509 (17.5 to 18.5 wt % Cr) and/or Europeandesignation 1.4760 (22 to 23 wt % Cr), although similar designations offerritic stainless steel may also be used, as would be understood by theperson skilled in the art.

The substrate may have a thickness in the range about 50 to 500 μm,often about 100 to 400 μm, in some cases about 200 to 350 μm. Thethickness of the substrate is determined by the need to provide a stablesubstrate, which doesn't warp during cell formation or in use, yet whichis as thin as possible to allow efficient contact between the fuel andthe anode. As described in GB 2 368 450, this contact can be achievedwith excellent results by the provision of a porous region bounded by anon-porous region of the substrate, over which the anode is formed. Itwill often be the case that the porous region of the substrate includesa plurality of through apertures fluidly interconnecting the one andother surface of the substrate, often these will be uniformly spaced,additionally or alternatively having a lateral dimension of from about 5to 500 μm, or from about 100 to 300 μm. Further, the apertures maycomprise from about 0.1 to 5 area % of the porous region of thesubstrate or from about 0.2 to 2 area % of the porous region of thesubstrate. Each of these features contribute to an efficient transfer offuel through the substrate to the anode, whilst allowing the metalsubstrate to support the fuel cell, facilitating the use of dramaticallyreduced thicknesses of the electrochemically active layers within thecell.

Typically the substrate will be a foil, although a sintered substratecould also be used. The advantage of foils is the ease of control of thestructure of the porous region.

The green anode layer is generally formed by application of an inkcomprising the nickel oxide, copper oxide and rare earth-doped ceria,although other methods may be used. These three components willgenerally be suspended as powders within an ink base, the ink basegenerally comprising one or more volatile solvents, one or moredissolved non-volatile polymer binders, dispersants, wetting agents andother common ink components. The nickel oxide, copper oxide and rareearth-doped ceria will often be of particle size distribution d90 in therange 0.1 to 4 μm, or 0.2 to 2 μm or 0.7 to 1.2 μm. Whilst the particlesize distributions, and sizes themselves, of each of the copper oxide,nickel oxide and rare earth-doped ceria may be different, it can bebeneficial if they are the same, or similar, as this helps to facilitategood mixing of the powders and hence strong sintering of the anode.Small particle sizes are generally selected as these are more easilysuspended in the ink, and offer a greater homogeneity of componentswithin the anode layer, and have a higher surface area to volume ratio,increasing the reactivity of the particles and ease of sintering.

Typically, the ink will contain in the range 30 to 70 wt % of the solidscontent in the ink of mixed metal oxides (namely, the combination ofcopper oxide and nickel oxide). Often, this will be 35 to 45 wt %, theremainder of the solids being the rare earth-doped ceria. That is tosay, it will often be the case that the only solids in the ink will bethe metal oxides and the rare earth-doped ceria, and as such it willoften also be the case that the anode consists of, or consistsessentially of, nickel oxide, copper oxide and the rare earth-dopedceria. Often, the metal oxide component of the ink will comprise in therange 5 to 50 wt % of the total metal oxide of copper oxide, often 8 to25 wt %. In many cases the copper oxide will be around 10 wt %, perhaps8 or 9 to 11 or 12 wt % of the total metal oxide, the ratio of nickeloxide to copper oxide therefore being around 9:1. The ratio willtypically be in the range 20:1 to 4:1, often in the range 15:1 to 6:1.It has been found that by selecting these levels of copper oxide doping,the relatively low melting point of the copper oxide offers enhancedsintering within the composite anode material, without lowering themetal oxide melting point below that necessary for sintering of the rareearth-doped ceria to occur, and without impairing anode functioning, inparticular where the fuel is hydrogen, or where the fuel cell is steamreforming hydrocarbons.

It will often be the case that the copper oxide is copper (II) oxide, asthis has a higher melting point than copper (I) oxide, and hassemi-conductor properties. However, copper (I) oxide may also be used asthis may form copper (II) oxide at high temperature in air.

In many examples, the rare earth-doped ceria will have the formulaCe_(1-x)RE_(x)O_(2-x/2), where RE is a rare earth and 0.3≧x≧0.05. Often,the rare earth-doped ceria will be gadolinium doped cerium oxide, oftenof the formula Ce_(0.9)Gd_(0.1)O_(1.95) (CGO10). These compounds aregenerally used as they have a higher oxygen ion conductivity than manyelectrolyte materials, including zirconia-based materials; therebyallowing operation of the fuel cell at lower temperatures thanconventional SOFCs, the temperature of operation of the fuel cell of theinvention typically being in the range 450° C. to 650° C., often 500° C.to 620° C. Operating the fuel cell at lower temperatures has a number ofbenefits, including reduced rate of oxidation of nickel in non-reducingatmospheres, which in turn often results in only the outer shell of theparticle oxidising, reducing volume change within the anode and hencerisk of cracking in the event that the reducing atmosphere of the fuelsupply is interrupted. Further, it makes the use of metal supportspossible, allowing thinner layers of electrode and electrolyte materialto be used, as these play less of a structural role, if any at all. Inaddition, these temperatures are well below the melting point of copper,providing the option of using copper as a component of the cell.

The application of the green anode layer generally includes an initialapplication of the ink to the metal substrate, this will typically be byprinting, for instance by screen printing, although other methods, suchas tape casting, vacuum slip casting, electrophoretic deposition andcalendering may be used as would be known to the person skilled in theart. Where a porous region is present, the application of the ink to thesubstrate will typically be such that a layer is formed over the porousregion, but the non-porous region is left substantially uncovered. Thisensures that the fuel cannot bypass the anode, but minimises materialcosts and weight by covering no more of the substrate than necessary.

This initial application will optionally be followed by a step of dryingthe ink to provide a printed layer. The drying may be air drying, orunder gentle heat. Gentle heat is often used to speed up the formationof the printed layer. Temperatures in the range 50° C. to 150° C. wouldbe typical. The drying step evaporates solvents and sets any binders inany ink formulation used, solidifying the ink and forming an initial,albeit fragile, anode layer, termed here the printed layer. This layerwill generally be of thickness in the range 5 to 40 μm, often 7 to 20μm, often 9 to 15 μm. As the fuel cells of the invention are not anodesupported cells, the anode layer can be much thinner than in manyconventional fuel cells, which has the advantage that the overall volumechange during REDOX cycling is smaller, and so cracking of the anodeover time is significantly reduced.

The applied nickel oxide, copper oxide and rare earth-doped ceria; orthe printed layer where a drying step is present, may then be heated toremove any organic components in an ink mixture, for instance, polymerbinders typically present in inks. The temperature of this step willdepend upon the binders present but will often be in the range 300 to500° C. This heating step may be combined with the drying step, althoughto provide a well formed, even, green anode layer the solvents aregenerally first removed, and then the organic components of the mixturein a separate step.

Often, where required, the compression step described above willtypically be applied after the ink has dried and the organics removed asat this stage the green anode layer comprises only the active components(namely the nickel oxide, copper oxide and rare earth-doped ceria). Thisallows the compression step to most efficiently compact the anode andincrease the density of the oxides and ceria so that sintering isimproved. A variety of compression methods may be used, as would beknown to the person skilled in the art, although often uniaxial or coldisostatic pressing will be used.

The step of firing the green anode layer to form a composite includingoxides of nickel, copper, and a rare earth-doped ceria provides forsintering of the rare earth-doped ceria and the metal oxides to form theceramic structure of the anode. Firing of the green anode layertherefore generally occurs in a furnace at a temperature in the range950 to 1100° C., often 980 to 1050° C. or 1000 to 1030° C. The upperlimit of these ranges is selected on the basis of substrate stability.Above around 1100° C. even high chromium content steels, known for theirhigh oxidation resistance, oxidise in air too rapidly for the substrateto survive the firing process. Specifically, the chromium oxidepassivation layer grows and flakes repeatedly during the formation ofthe anode cermet, weakening the metal substrate to an unacceptableextent. The use of the rare earth-doped ceria facilitates the use of ametal substrate, together with the formation of a robust cermet as ceriacompounds may be sintered at temperatures below 1100° C. The lower limitis guided by the need for successful sintering of the materials.

The firing step will typically be firing in air, although othernon-reducing atmospheres may be used. Typically the firing step will beover a period 15 to 60 minutes. Whilst the firing period must besufficient to allow sintering of the metal oxides and the rareearth-doped ceria, and to allow the furnace to reach thermalequilibrium; too long a firing period can increase oxidation of themetal support and lead to contamination of the anode with, whereferritic stainless steel is used, chromium evaporating from the support.Hence, the optimal firing period is in the range 15 to 60 minutes. Aftersintering the anode is allowed to cool, providing a robust, porous,anode structure containing nickel, copper and the rare earth-doped ceriaas an ceramic oxygen ion conductor.

Whilst as described above the firing of the anode occurs before theelectrolyte is provided, it may be that the electrolyte be applied overthe green anode layer before firing occurs. As such, the process maycomprise the step of providing an electrolyte before the step of firingthe green anode layer, so that the electrolyte and green anode layer aresimultaneously fired.

Typically, the electrolyte for use with the fuel cells of the inventionwill be of thickness in the range 5 to 30 μm, often in the range 10 to20 μm. The provision of such a thin electrolyte layer provides for rapidtransfer of oxygen ions from the cathode, to the anode. Often theelectrolyte will comprise a rare earth-doped ceria, appropriate rareearth cerias being as defined above for the anode. In some examples, theelectrolyte may comprise a rare earth-doped ceria combined with a lowlevel of cobalt oxide and/or copper oxide, as a sintering aid, forinstance there may be in the range 0.5 to 5 wt % cobalt oxide and/orcopper oxide, the remaining electrolyte being the rare earth-dopedceria. The use of rare earth-doped cerias for both the anode andelectrolyte helps to enhance the compatibility between these componentsof the fuel cell both chemically and in terms of the thermal expansion,which is closely matched reducing the mechanical stress between layersduring REDOX cycling, and hence also reducing the likelihood of crackingand fuel cell failure in use. Further, as these cerias have high chargetransfer rates, their inclusion ensures a good rate of charge transferbetween the electrolyte and the anode.

The electrolyte will generally be sintered, either simultaneously withthe anode as described above, or in a separate firing step after theanode is fully formed.

Typically the cathode will be of thickness in the range 30 to 60 μm,often 40 to 50 μm. The cathode will generally comprise two layers, athin active layer where the reduction of oxygen takes place, and athicker current collector layer, to allow the current to be collectedfrom a cell in the stack. The current collector layer will generally bea perovskite such as lanthanum strontium cobaltite, although anyelectronically conductive ceramic material may be used.

The active layer cathode may comprise a sintered powdered mixture ofperovskite oxide mixed conductor and rare earth-doped ceria, the rareearth-doped ceria being as defined above. The perovskite may compriseLa_(1-x)Sr_(x)Co_(y)Fe_(1-y)O₃₋₈, where 0.5≧x≧0.2 and 1≧y≧0.2. Inparticular, the perovskite oxide mixed conductor may comprise one ormore of La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃₋₆, Gd_(0.5)CoO₃₋₆, andRE_(x)Sr_(1-x)CoO_(3-d), (where RE=La, Sm, Pr and 0.5<x<0.8). It can beuseful to use these compounds as they have a higher ionic conductivitythan most perovskites. In some cases, the mixture comprises in the range20 to 50 wt % rare earth-doped ceria, in some cases 30 to 45 wt %, insome cases 35 to 45 wt %, or around 40 wt % rare earth-doped ceria asdefined above. This helps to enhance the compatibility between thecathode and electrolyte both chemically and in terms of the thermalexpansion described above, and as these cerias have high charge transferrates, their inclusion ensures a good rate of charge transfer betweenthe electrolyte and the cathode.

The cathode will generally be sintered before use. The cathode willtypically be applied as one or more layers (for instance active andcurrent collecting) directly or indirectly over the sintered electrolyteand sintered under conditions similar those described above for theanode. This provides an intermediate temperature metal supported SOFC,which is robust to repeated REDOX cycling, and as a result of the anodestructure formed, to fuel depravation whilst at high temperature.

In a second aspect of the invention there is provided a metal supportedSOFC comprising an anode, a cathode and an electrolyte, wherein theanode includes nickel, copper and a rare earth-doped ceria. As describedabove, to provide a robust fuel cell structure, the nickel, copper andrare earth-doped ceria are generally sintered. In use, the nickel may bein a form comprising metallic nickel, nickel oxide and combinationsthereof; depending upon the REDOX state of the nickel. For instance, thenickel will be in the form of nickel oxide upon formation of the cell,but will be reduced to nickel metal at the point of first use of thecell. Similarly, the copper may be in a form comprising metallic copper,copper (I) oxide, copper (II) oxide and combinations thereof. Further,mixed metal and metal oxide phases may be formed, due to the mutualsolubility of nickel and copper at high temperatures. As such, it may bethe case that a nickel-copper alloy is formed, which when oxidised formsa nickel-copper mixed metal oxide, which could be generally described ashaving the formula Ni_(x)Cu_(1-x)O, with x being variable between 0 and1 as would be understood by the skilled reader. Mixed oxides containingcerium could also be formed, due to the solubility of copper oxide indoped ceria. The anode, cathode, and electrolyte will, in otherrespects, be generally as described above.

In some instances, the fuel cell will be a fuel cell of the typedescribed in the applicants granted patent GB 2 368 450, which isincorporated herein by reference. In such cases, the fuel cell maycomprise:

(i) a ferritic stainless steel support including a porous region and anon-porous region bounding the porous region;

(ii) a ferritic stainless steel bi-polar plate located under one surfaceof the porous region of the support and being sealingly attached to thenon-porous region of the support about the porous region thereof;

(iii) an anode comprising an anode layer located over the other surfaceof the porous region of the support;

(iv) an electrolyte comprising an electrolyte layer located over theanode layer; and

(v) a cathode comprising a cathode layer located over the electrolytelayer;

wherein the anode includes nickel, copper and a rare earth-doped ceria.

The fuel cell may be present in a fuel cell stack, comprising two ormore fuel cells, and there is therefore provided in a third aspect ofthe invention, a fuel cell stack comprising fuel cells according to thesecond aspect of the invention. Each fuel cell may comprise a bi-polarplate, as described above, to which the support may be welded, orotherwise sealed.

In a fourth aspect of the invention, there is also provided for the useof a fuel cell according to the second aspect of the invention in thegeneration of electrical energy.

The process of the invention is intended to provide a method for themanufacture of a highly sintered nickel-copper-rare earth-doped ceriathick film anode suitable for use in a metal supported SOFC cell, whilstavoiding the problems of poor anodic sintering, and delamination of theelectrolyte in use. It may be the case that the process is a process forforming a metal supported solid oxide fuel cell, the process comprisingthe steps of:

a) applying a green anode layer including nickel oxide, copper oxide anda rare earth-doped ceria (optionally powdered) to a metal substrate,wherein the powders are optionally of particle size distribution d90 inthe range 0.2 to 3 μm and wherein the nickel oxide, copper oxide andrare-earth doped ceria are optionally applied as an ink, the inkoptionally comprising a total solids content in the range 30 to 70%mixed metal oxides, with optionally in the range 5 to 50 wt % of thetotal metal oxide of copper oxide;

b) optionally drying the ink to provide a printed layer of thickness inthe range 5 to 40 μm;

c) optionally compressing the green anode layer at pressures optionallyin the range 100 to 300 MPa;

d) optionally, heating the printed layer to remove the ink base leavinga green anode layer comprising nickel oxide, copper oxide and a rareearth-doped ceria;

e) firing the green anode layer at a temperature optionally in the range950 to 1100° C. to form a composite;

f) providing an electrolyte; and

g) providing a cathode.

Unless otherwise stated each of the integers described in the inventionmay be used in combination with any other integer as would be understoodby the person skilled in the art. Further, although all aspects of theinvention preferably “comprise” the features described in relation tothat aspect, it is specifically envisaged that they may “consist” or“consist essentially” of those features outlined in the claims. Inaddition, all terms, unless specifically defined herein, are intended tobe given their commonly understood meaning in the art.

Further, in the discussion of the invention, unless stated to thecontrary, the disclosure of alternative values for the upper or lowerlimit of the permitted range of a parameter, is to be construed as animplied statement that each intermediate value of said parameter, lyingbetween the smaller and greater of the alternatives, is itself alsodisclosed as a possible value for the parameter.

In addition, unless otherwise stated, all numerical values appearing inthis application are to be understood as being modified by the term“about”.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood, itwill be described further with reference to the figures and to thespecific examples hereinafter.

FIG. 1 is a schematic representation of a SOFC as described in GB 2 368450;

FIG. 2 is a scanning electron micrograph (SEM) showing a cross sectionthrough a SOFC of FIG. 1 (15.0 kV, 7.9 mm×1.50 k);

FIG. 3 is a SEM showing a cross section through a SOFC including ananode formed from a nickel oxide-copper-oxide-GCO composite (15.0 kV,8.5 mm×1.0 k);

FIG. 4 is a SEM showing a cross section of a sintered anode formed froma nickel oxide-GCO composite (15.0 kV, 7.1 mm×13 k);

FIG. 5 is a SEM showing a cross section of a sintered anode formed froma nickel oxide-copper-oxide-GCO composite (15.0 kV, 8.9 mm×13 k);

FIG. 6 is a current-voltage curve for the SOFC of FIG. 3 as a functionof cell operating temperature (56% hydrogen-44% nitrogen fuel, excessair fed to cathode);

FIG. 7 is a power-cycle graph of the SOFC of FIG. 3; and

FIG. 8 is a table showing the enhanced mechanical strength of the nickeloxide-copper-oxide-GCO composite as illustrated in FIG. 3 relative tothe nickel-CGO anode illustrated in FIG. 2.

DETAILED DESCRIPTION

A SOFC 10 as described in GB 2 368 450 is shown schematically in FIG. 1,and in SEM cross-section in FIG. 2. Both figures show a ferriticstainless steel substrate 1, made partially porous by laser-drillingthousands of holes though the central region of the substrate 2. Theporous substrate is covered by a nickel oxide and CGO anode layer 3covering the porous region 2 of the substrate 1. Over the anode layer 3is deposited a CGO electrolyte layer 4 (10-20 μm, CGO), which overlapsthe anode 3 onto the undrilled area 9 of the substrate 1, thus forming aseal around the edge of the anode 3. The cathode 5,6 has a thin activelayer 5 (CGO composite) where the reduction of oxygen takes place, and athicker current collector layer 6 (lanthanum strontium cobaltite) toallow current to be collected from the cell 10 in a stack. FIG. 2additionally shows a very thin stabilised zirconia layer 7 and an eventhinner doped ceria layer 8, which block electronic conductivity(preventing short circuiting from undesirable chemical reactions betweenthe cathode 5,6 and zirconia layer 7) and form the interface between theanode 3 and electrolyte 5,6 respectively.

SOFC 10 of FIGS. 1 and 2 was prepared by applying a screen-printing inkcontaining suspended particles of nickel oxide powder and CGO powder(d90=0.7 to 1.2 μm, ratio of nickel oxide to CGO in the ink being 1.8:1by weight). The ink was screen printed onto ferritic stainless steelsubstrate 1 using conventional methods, and dried in an oven toevaporate the solvents and set the binders thereby forming a dried,printed layer of thickness 9 to 15 μm. The dried, printed layer wascompressed using cold isostatic pressing at pressure of 300 MPa. Thegreen anode layer was placed in a furnace and heated to a temperature of960° C. in air atmosphere for 40 minutes, to produce a sintered anodelayer 3. A CGO electrolyte layer 4 was sprayed onto the anode layer 3and fired in a furnace at 1020° C. for 40 minutes. Finally, zirconialayer 7 was applied to the fired electrolyte layer by means of themethod disclosed in GB 2 456 445 followed by application of the dopedceria layer 8 and the two cathodic layers 5,6 also using the methods ofGB 2 456 445, before firing at a temperature of 825° C. to produce theSOFC 1 structure.

FIG. 3 shows a cross-section through a SOFC including nickeloxide-copper oxide-CGO composite as claimed. The nickel oxide and copperoxide are present in a 9:1 ratio by weight resulting in a 9:1 ratio ofnickel to copper in use. Subject to the introduction of copper into theanodic structure such that the 1:1.3 ratio of nickel oxide:CGO describedabove becomes a 1:1.3 ratio of the mixed metal oxide (namely nickeloxide and copper oxide) to CGO, the structure of the fuel cell was inaccordance with the prior art cell of FIGS. 1 and 2. The manufactureclosely followed the preparation method of the prior art cell, with theexception that the dried printed layer was heated in an oven to atemperature of 350° C. prior to compression to remove the organicbinders in the ink and provide a green anode layer, and that the firingof the anode was at 1020° C. for 45 minutes.

Examples Anode Structure

FIGS. 4 and 5 show the difference in anode structure obtained throughthe addition of copper oxide to the composite structure. The compositeof FIG. 4 has the composition 64 wt % nickel oxide to 36 wt % CGO andthe composite of FIG. 5, 51 wt % nickel oxide, 5.7 wt % copper (II)oxide and 43.3 wt % CGO. In order to improve the REDOX stability of thenickel-copper anode in FIG. 5, the level of metal oxide was reducedsomewhat relative to the original anode shown in FIG. 4. After reductionduring fuel cell operation, the anode cermet in FIG. 4 is 53 vol % metalas opposed to 45 vol % metal in FIG. 5. It has been shown that reducingthe metal content alone does not confer adequate REDOX stability; theaddition of copper is required as well. Both composites were prepared asabove, and fired in air at 1020° C. for 60 minutes before fabricationinto cells and reduction to metal in situ to form the cermets shown.

Good sintering is evidenced by a clear distinction between ceramic andmetallic regions, and by the particles of both ceramic and metallicphases having fused together. The ceramic regions appearing as lightregions and the metallic regions as dark patches. As can be seen, thecomposite of FIG. 5, which contains copper, includes larger, darkermetal particles, indicating good sintering, the well sintered structureof the CGO is also readily apparent. This well sintered structure canalso be seen in FIG. 3 (anode 3).

The resulting anode structure has been demonstrated to be highlyREDOX-stable at operating temperatures of <650° C., being capable ofwithstanding hundreds of high-temperature fuel interruptions withoutsignificant cell performance degradation.

Selection of Copper

A range of cations are known to enhance doped ceria sintering, theseinclude copper, cobalt, iron, manganese and lithium (U.S. Pat. No.6,709,628, J. D. Nicholas and L. C. De Jonghe, Solid State Ionics, 178(2007), 1187-1194). Consideration was therefore given to doping the rareearth-doped ceria with one of these cations. Of the above cations,copper, cobalt and lithium are reported to the most effective atenhancing the sintering of rare earth-doped ceria. Copper and cobalt arethe only cations considered by the applicant to be suitable for use inan SOFC anode as lithium oxide is highly reactive, and in addition isknown to be very detrimental to the ionic conductivity of rareearth-doped ceria by forming an insulating phase on the grainboundaries. Cobalt is well known to enhance the sintering of rareearth-doped ceria, and in addition is known to be effective as an anodecatalyst (C. M. Grgicak, R. C. Green and J. B. Giorgi, J. Power Sources,179(1), 2008, 317-328), although typically less so than nickel. Howeverinitial evaluation of the sintering behaviour of composites using apush-rod dilatometer surprisingly demonstrated that cobalt oxide isineffective in enhancing the sintering of nickel oxide, and thus thesinterability of the anode composite was not significantly enhanced bythe partial or even complete substitution of nickel oxide with cobaltoxide. Copper oxide by contrast demonstrated a great increase in thesinterability of the composite, partly it is suspected because it mayform a low melting-point eutectic with nickel oxide, thus introducingsome liquid-phase sintering.

Fuel Cell Performance

FIG. 6 is a series of current-voltage polarisation curves for the fuelcell of FIG. 3, at different operating temperatures. Fuelling rate wascalculated to give approximately 60% fuel utilisation at 0.75V/cell ateach of the measured temperatures, showing that the system can beoperated across a range of temperatures at least as broad as 495 to 616°C., allowing the operational temperature to be optimised forapplication, number of cells in the stack, output required etc.

FIG. 7 shows the very good REDOX stability possible with this anodestructure. A series of cycles are run at 600° C. on a seven-layer shortstack, where a current-voltage curve is run to establish the stackperformance. The stack is then returned to open circuit, and thehydrogen supply to the stack is cut whilst maintaining the stack at580-600° C. Air and nitrogen are maintained to the stack during thisperiod. The fuel interruption is sustained for 20 minutes, allowing timefor the anode to partially reoxidise. The hydrogen feed is thenrestored, and after giving the stack a few minutes to recover, anothercurrent-voltage curve is run to determine if stack performance has beenlost as a result of the REDOX cycle of the anode. This sequencecontinues until stack performance starts to fall, indicating damage toone or more cells as a result of REDOX cycling.

It can be seen from FIG. 7 that with the SOFC cell of FIG. 3, the sevencells within the stack will tolerate more than 500 REDOX cycles withoutany significant loss of performance after a small initial burn-in, with544 cycles being run in total.

Enhanced Mechanical Strength of Anode Resulting from Copper Addition

FIG. 8 is a table of the results of mechanical strength tests undertakenon SOFC cells both after initial manufacture and after cells haveoperated in an initial performance characterisation test, for bothstandard nickel-CGO anodes as illustrated in FIG. 2, andnickel-copper-CGO anodes as illustrated in FIG. 3.

In the as-manufactured cells, the anodes are in the oxidised state andprior to the mechanical test they are reduced in order to mimic theanode structure in the cell at the start of operating, whereas theanodes in the “after operating” cells are in the final cermet state ofthe working anodes.

In order to perform the mechanical strength measurement on the cells,the metal substrates of the cells are first glued to a flat steel plateto prevent the cells flexing when a pulling force is applied. Thecathodes of the cells are removed mechanically, exposing theelectrolyte.

To assess the mechanical strength of the anode and/or theanode-electrolyte bond, circular metal test pieces are glued to theelectrolyte surface in the four corners of the electrolyte and themiddle of the cell. A diamond scribe is used to cut through the ceramiclayers of the cell around the metal test piece. A calibrated hydraulicpuller is then attached to the test piece and used to measure the stressrequired to pull the test piece off the cell substrate. A maximumpulling stress of 17 MPa may be applied using this technique, afterwhich the glue holding the test piece to the electrolyte tends to failrather than the fuel cell layers on test. Should the test piece bepulled off at less than 17 MPa this indicates the failure stress of theweakest cell layer (usually the internal structure of the anode).

It can be seen that whilst the standard nickel-CGO anodes are strong inthe as-manufactured state, they fail at much lower stresses afterreduction of the nickel oxide to metallic nickel in the “afteroperating” cell. Without being bound by theory, it is believed this islargely because of the lack of a contiguous ceramic structure within theanode, meaning the mechanical strength of the anode is provided entirelyby relatively weak necks between nickel particles. By contrast it can beseen that the nickel-copper CGO anodes retain their strength afterreduction to the cermet structure, indicating much greater sintering ofboth metallic and ceramic phases.

It should be appreciated that the processes and fuel cell of theinvention are capable of being incorporated in the form of a variety ofembodiments, only a few of which have been illustrated and describedabove.

1. A process for forming a metal supported solid oxide fuel cell, the process comprising the steps of: a) applying a green anode layer including nickel oxide, copper oxide and a rare earth-doped ceria to a metal substrate; b) firing the green anode layer to form a composite including oxides of nickel, copper, and a rare earth-doped ceria; c) providing an electrolyte; and d) providing a cathode.
 2. The process according to claim 1, further comprising a step of compressing the green anode layer at pressures in the range 100 to 300 MPa.
 3. The process according to claim 1, wherein the firing of the green anode layer occurs at a temperature in the range 950 to 1100° C.
 4. The process according to claim 1, wherein the nickel oxide, copper oxide and rare earth-doped ceria are powdered, the powders being of particle size distribution d90 in the range 0.1 to 4 μm.
 5. The process according to claim 1, wherein the nickel oxide, copper oxide and rare earth-doped ceria are applied as an ink.
 6. The process according to claim 5, wherein the ink comprises in the range 5 to 50 wt % of the total metal oxide of copper oxide.
 7. The process according to claim 6, wherein the application of the green anode layer includes an initial application of the ink to the metal substrate, and drying the ink to provide a printed layer of thickness in the range 5 to 40 μm.
 8. The process according to claim 1, further comprising heating the printed layer to remove the ink base leaving a green anode layer comprising nickel oxide, copper oxide and a rare earth-doped ceria.
 9. The process according to claim 1, wherein the step of providing an electrolyte occurs before the step of firing the green anode layer, so that the electrolyte and green anode layer are simultaneously fired.
 10. A metal supported solid oxide fuel cell comprising an anode, a cathode and an electrolyte, wherein the anode includes nickel, copper and a rare earth-doped ceria.
 11. The fuel cell according to claim 10, wherein the nickel, copper and rare earth-doped ceria are sintered.
 12. The fuel cell according to claim 10, wherein a weight ratio of nickel oxide to copper oxide is in the range 20:1 to 4:1.
 13. The fuel cell according to claim 10, wherein the nickel is in a form selected from metallic nickel, nickel oxide, a nickel-copper alloy, a nickel-copper oxide and combinations thereof.
 14. The fuel cell according to claim 10, wherein the copper is in a form selected from metallic copper, copper (II) oxide, copper (I) oxide, a nickel-copper alloy, a nickel-copper oxide and combinations thereof.
 15. The fuel cell according to claim 10, comprising: (i) a ferritic stainless steel support including a porous region and a non-porous region bounding the porous region; (ii) a ferritic stainless steel bi-polar plate located under one surface of the porous region of the support and being sealingly attached to the non-porous region of the support about the porous region thereof; (iii) an anode comprising an anode layer located over the other surface of the porous region of the support; (iv) an electrolyte comprising an electrolyte layer located over the anode layer; and (v) a cathode comprising a cathode layer located over the electrolyte layer; wherein the anode includes nickel, copper and a rare earth-doped ceria.
 16. The fuel cell according to claim 10, wherein the rare earth-doped ceria comprises gadolinium doped cerium oxide.
 17. A fuel cell stack comprising two or more fuel cells according to claim
 10. 